The Hidden Forest

Ifir is backstabbing max for 1004 as the 2.0 is backstabbing max around 2446. Also the backstab mods on the 2.0, Ifir, Salindrite Dagger, and 1.5 are all 15% while Mrylokar's Dagger of Vengeance(Vulak) is 20%

List of all the daggers I can think of off the top of my head:

Willsapper
Ragebringer
Chaotic Serrated Bone Dirk
Fierce Piercer of the Scarab
Ancient Tool of the Thieves
Bloodbath